The present invention relates to a gas flow determination method. The present invention also relates to a corresponding gas flow determination device.
Although applicable to any desired gas flows, the present invention and the problems on which it is based are explained here with respect to an air flow meter on board a motor vehicle.
FIG. 2 shows a cross section through a conventional gas flow determination device in the form of such an air flow meter.
This air flow meter includes a sensor device SD including a diaphragm D and a frame FR enclosing it. Sensor device SD is manufactured in one piece with diaphragm D and frame FR enclosing it by silicon micromechanical technology. C denotes a mount in the form of a carrier plate.
In addition, a temperature profile generating device is provided to create a predetermined temperature profile on diaphragm D.
A heating device is used for heating a central region H of diaphragm D to a predetermined temperature to create a trapezoidal temperature profile on diaphragm D which includes a plateau in the central region dropping linearly toward the edge region.
Curve A of the standardized temperature profile shown above that indicates the status of the temperature profile without oncoming flow. Curve B indicates the status of the temperature profile with oncoming flow. The oncoming flow with air thus yields a change in the temperature profile on diaphragm D in the region which drops linearly toward the edge region without oncoming flow. This change originates from the heat transport of the oncoming gas in direction of flow F.
A detection device T1, T2 composed of two temperature sensors in the form of Pt resistors is configured so that it detects the change in temperature dT1 caused by the gas-flow at a first location on diaphragm D upstream from heated central region H and the change in temperature dT2 caused by the gas flow at a second location on diaphragm D downstream from heated central region H.
In addition, a determination device (not shown) is also provided and is configured so that it determines the gas flow on the basis of the difference between the temperature changes thus detected, this difference being given by xcex94T=dT1xe2x88x92dT2.
FIG. 3 shows a schematic diagram of soiling on the conventional gas flow determination device in the form of an air flow meter.
In the case of the air flow meter illustrated in FIG. 2, there are deviations in the characteristic lines due to asymmetrical soiling S1 on diaphragm D. Due to soiling S1 shown here on the oncoming flow side, the temperature is lowered there due to the better thermal bonding at the edge of the diaphragm (lower heat resistance). In addition, there may be altered thermal coupling to the oncoming flow gas in the soiled region.
The gas flow determination method according to the present invention may provide the advantage that the effect of soiling on the diaphragm may be minimized effectively.
The idea on which the present invention is based is that the temperature at a third location on the diaphragm upstream from the first location and optionally at a fourth location downstream from the second location is regulated at a value corresponding to the local value of the predetermined temperature profile. Regulation at the third location is more important than regulation at the fourth location because greater deviations are to be expected at the third location than at the fourth location.
According to an example embodiment, the central region of the diaphragm is heated to a predetermined temperature to create a trapezoidal temperature profile on the diaphragm which includes a plateau in the central region dropping linearly toward the edge region.
According to another example embodiment, the change in temperature caused by the gas flow is detected at a first location on the diaphragm upstream from the heated central region and at a second location on the diaphragm downstream from the heated central region, and the gas flow is also determined on the basis of the difference between the temperature changes thus detected.
According to another example embodiment, a central region of the diaphragm is heated to a predetermined temperature above the temperature of the gas flow to be detected.
According to another example embodiment, the third and fourth locations are selected so that they are located in the edge region of the diaphragm.
Example embodiments of the present invention are illustrated in the drawings and explained in greater detail in the following description.